Voltage controlled oscillators (VCOs) generate a periodic clock signal in response to an input voltage. There are numerous applications for VCOs such as in tunable frequency generators. Additionally, VCOs are often used in phase locked loop (PLL) circuits to generate an output signal that is dynamically phase and frequency compared to an input reference signal. To keep the generated output signal aligned with the input reference signal, the phase difference is monitored and a correction circuit is used to change either the phase or the frequency (or both) of the generated clock signal to match the phase and frequency of the reference clock. Designers of VCOs have been faced with the problem of providing a relatively small and easy-to-implement phase correction circuit that will operate at low power supply voltages (e.g. 1.0-1.5 volts) while having a wide range of frequency operation and a wide range of phase correction.
Therefore, it is desirable to provide an integrated circuit VCO having a phase correction circuit that is small and easy to implement.